This invention is in the field of printing, and is more specifically directed to high-speed industrial ink jet printing.
As a result of continuing technological advances in the field, ink jet printing has become more popular in recent years, over a wider range of printing applications. For example, color ink jet printers with very high resolution (on the order of 600 dpi), and thus capable of photographic quality output, are now available even at consumer prices.
Ink jet printing is now also becoming popular in industrial applications. As mentioned above, ink jet printers provide excellent resolution at relatively low cost, and are especially attractive for the printing of small run jobs. Ink jet printers also can provide a great deal of flexibility in the printing of a wide range of formats. More specifically, ink jet printers appear especially attractive for wide format output (e.g., eighteen inches or wider), because electrographic or offset printing equipment for such wide format output is extremely costly.
However, high resolution and accuracy in ink jet printing requires not only small dot pitch for the ink as dispensed, but also close spacing between the ink jet printhead and the receiver sheet to minimize errors in ink drop placement (primarily due to variations in the angle of drop ejection from the printhead). Preferably, the printhead-to-receiver gap should be on the order of 1 mm or less. In conventional ink jet printers, however, such close spacing often results in contamination to the ink jet orifices from dust carried by the receiver, or from fibers of the receiver itself. The ink jet printhead may even become damaged by raised areas of the receiver itself, or by contaminants at the receiver surface, that actually touch the ink jets as the receiver passes by, especially in high-speed printers. In the industrial printing context, the control of this precise printhead-to-receiver spacing over the desired wide-format receiver width is also very difficult.
By way of background, U.S. Pat. Nos. 5,372,852; 5,389,958; 5,777,650; 5,864,774; 5,974,298; 6,102,538; 6,113,231; and 6,196,675 B1 describe ink jet transfer printing. In these references, ink jets dispense phase change ink, in the form of the image to be printed, onto an intermediate medium. The references disclose various types of intermediate media, examples of which include a roller, a web, and a belt, and also include a liquid intermediate surface disposed on such members. According to these references, the ink is transferred from the intermediate transfer surface to the receiver sheet by heat, or by the combination of heat and pressure.
U.S. Pat. No. 6,390,617 B1 describes some of the problems with this conventional phase-change ink jet transfer technique. As described in this reference, in some conventional approaches, the inked intermediate medium is heated, so that transfer to the receiver is effected by pressure of the heated intermediate medium against the receiver. The heating of the intermediate medium can result in expansion of the ink droplets, causing a tendency of the image to lose its shape on transfer. Also as described in this reference, some conventional approaches heated the receiver from the backside, rather than heating the intermediate medium. According to this reference, this approach requires excess heat and time, can cause shrinking or deforming of the receiver medium, and makes duplex printing problematic. This reference also discloses the heating of only the image side of the receiver, at transfer of the phase-change ink from the intermediate medium to the receiver.
In these hot-melt ink jet printers, either or both the receiver or the intermediate transfer surface must be heated. It is believed, in connection with this invention, that this heating tends to cause the ink to spread on transfer, resulting in degraded resolution and poor image fidelity. In addition, it is believed that the dispensed ink on the intermediate medium in these hot-melt ink jet printers is not completely cooled, and thus stays at least partially liquid while on the intermediate medium. For example, U.S. Pat. No. 6,196,675 B1 discloses that its ink droplets are dispensed onto a liquid intermediate transfer surface; in addition, while this reference discloses that its droplets are then cooled on the liquid intermediate transfer surface, the ink remains at an intermediate temperature so that the ink is in a “malleable” state. This instability in the ink is also believed to be subject to ink spreading, especially when different color inks are sequentially dispensed onto the intermediate transfer surface.
By way of further background, U.S. Pat. No. 6,279,474 B1 describes offset printing machines in which ink is initially delivered by ink jets to an ink form roller, which in turn transfers the ink it receives to the plate cylinder of the offset printing machine. U.S. Pat. No. 6,427,591 B1 describes offset printing machines in which ink jets deliver ink to an application roller or to a rotating mantle surface, which in turn delivers the ink to an application roller and ultimately to the plate cylinder. In each case, the ink jets permit close control of the amount of ink delivered to specific “zones” of the printed output, without requiring a complex sequence of braying rollers, blades, and the like.
By way of further background, the electrostatic transfer of polymer particles, using corona charge mechanisms, is well known in connection with conventional laser printers and copiers. As is well known in this art, the image is defined by the selective exposure of a charged photoconductor, for example by a raster-scanning laser. Toner ink particles are then attracted to the photoconductor in a pattern corresponding to the exposure of the image. The toner is then electrostatically transferred to a receiver, and fused using heat and pressure.
By way of further background, U.S. Pat. No. 6,126,274 describes a method of indirect printing in which toner particles, suspended in a dielectric fluid, are agglomerated and then dispensed by ink jets to an intermediate image holder in the form of an image to be printed. The toner particles are then electrostatically transferred to the receiver sheet, and the image is fused by heat and pressure.
By way of still further background, U.S. Pat. No. 6,682,189 B2 describes a method of indirect printing in which aqueous and non-aqueous inks, in the form of colloidal dispersions of pigment, are ink-jetted to form a coagulable ink image on an intermediate member, such as a roller or web. A coagulate formation process is performed on the jetted ink, and the liquid of the coagulated dispersion is then removed. The image is transferred to a receiver, with electrostatic and thermal transfer processes disclosed.